In a hydraulic elevator, the car is supported by a hydraulic jack, which is used to raise and lower the car between floors. To raise the car, a pump supplies oil to a hydraulic control valve, which directs pressurized oil into the jack to raise the car to the desired floor. Usually the same hydraulic control valve is used to lower the car. In this case, however, it is not necessary to start the pump, in that the valve merely vents oil from the plunger back to the tank.
For up runs, the valve, rather than the pump motor, is used to control car movement. When the pump motor, which is typically an AC induction motor, is started, the valve initially bypasses all of the oil from the pump back to tank. After the pump motor has essentially reached full speed, the valve reduces the amount of bypass so that part of the pressurized oil is directed toward the jack, causing the jack, and the car, to begin to rise. Thereafter, the valve normally reduces further the amount of bypass, directing more oil to the jack and thereby increasing car speed. This may be done either in discrete steps or using a variable control.
Because the car is supported by a column of oil, a car that is initially level with the floor may not remain level. Oil is compressible, and changes in load inside the car (as passengers enter and leave the car) can cause the car to move away from the landing. Oil in the column may cool when the car is parked at a landing, causing the oil to contract. Oil leakage can also cause the car to sink.
It is therefore necessary to provide a relevel function to prevent the car from moving more than a prescribed distance away from the landing. This is usually done by providing sensors that detect when the car has moved a certain distance away from the landing. If the car sinks more than a desired distance below the landing, a relevel up operation is carried out. If the car moves upwardly more than a prescribed distance, a level down operation is effected.
Re-level up operations are carried out in a manner similar to other up runs. The pump motor is accelerated to full speed, at which time the hydraulic valve directs a portion of the pressurized oil to the jack to return the car slowly back to level. Once the car is again level, the motor is deenergized and allowed to coast to a stop.
Thus, it is necessary to start the motor, and run it to full speed, every time a relevel is required in the up direction. Appreciable heat is produced in the motor during start because the average slip is very high. Since the motor runs for only a few seconds, the internal fan is ineffective to remove the heat produced during a start. Therefore, any condition that causes an excessive rate of releveling operations increases the likelihood of the motor overheating and eventually failing. Hydraulic pump motors are rated for a maximum number of starts per hour. This rate can easily be exceeded by cyclic releveling.
The low car speed required for releveling is obtained by supplying only a portion of the pressurized oil to the jack, and bypassing most of the pump output back to tank. The oil is heated during each bypass operation. As a result, the oil temperature is raised during each releveling operation. Higher oil temperatures can reduce the efficiency of the pump and thereby waste energy.
Current relevel systems utilize a pair of sensors located at set distances above and below the landing, which define a certain "dead zone". A typical distance of the dead zone is currently 7/8 inch. If the car moves far enough from the landing to engage the upper or lower dead zone sensor, a releveling operation will be initiated.
The switches which initiate a relevel operation have a certain amount of hysteresis inherent in the construction. As a result, when the sensor causes actuation of the switch, and the car starts to move back towards level, the switch remains closed for a certain period of time. The delay, which may be the result of friction and inertia, will permit the car to keep moving toward level until it is some distance away from the sensor. Moreover, the start needle and stop needle on the hydraulic valve can be manually adjusted to produce an acceptable releveling operation for any reasonable hysteresis value. Thus, between the inherent hysteresis value of the releveling switch, and a manual adjustment of the hydraulic valve, the distance of movement during a relevel operation can be selected so that the car is returned to the floor level position. If the dead zone is subsequently made larger or smaller, however, the valve must either be readjusted or the hysteresis of the sensor must be changed.
As discussed above, changes in load in the car, a change in oil temperature, and leakage, may all cause the car to move away from the landing and initiate a relevel operation. If the sensor that initiated a level up operation has insufficient hysteresis the valve will begin to close again before the car has moved appreciably. Insufficient hysteresis in the leveling sensor can therefore contribute to cyclic releveling. Moreover, while a narrow dead zone is desirable from a performance standpoint, a narrow dead zone is also a major cause of excessive releveling.